Aqueous rechargeable batteries generate gas, at the conclusion of charge and during overcharge, after the active electrodes become fully charged. In a sealed cell and battery configurations, the rate of charge needs to be controlled to avoid excessive buildup of pressure within the cells that can cause cell rupture and damage. Alkaline rechargeable batteries of, for example, nickel, nickel cadmium, nickel zinc, nickel-iron and nickel metal hydride type may utilize cells in which excess negative anodic material is designed into the cell. In this configuration, oxygen gas may be generated on the surface of the nickel electrode at the end of charge before hydrogen starts to generate on the negative electrode.
The charge reaction of the Nickel electrode isNi(OH)2+OH—→NiOOH+H2O+e−
The evolution reaction of Oxygen is4OH—→O2+2H2O+4e−
The charge reaction of the Hydride electrode is:M+H2O+e−→MH+OH—
In a limited electrolyte cell design, the oxygen generated during overcharge can be recombined on the surface of the negative electrode avoiding excessive pressure buildup within the cell.
The recombination reaction of Oxygen on the Hydride electrode is:2MH+½O2→2M+H2O
The rate of recombination may be based on mass transport or kinetics in a sealed cell. For example, it may be dependent on the catalytic activity of the oxygen on the negative electrode and on the access of oxygen to the surface of the negative electrode. The rate of recombination may be pressure and/or temperature dependent. In typical cells, the steady state recombination may be limited to the five to ten hour charge rate. Furthermore, the overcharge recombination process can generate heat within the cells that may damage cell materials and lead to thermal runaway.
Therefore, many techniques have been developed to control and limit the charge of batteries to avoid excessive heating and/or damage from generation of excessive internal pressures. Techniques that are utilized to control charge include, for example, the monitoring and control of temperature, the rate of change of temperature and/or cell voltage, the rate of change of cell voltage and/or pressure, the rate of change of pressure, and/or any combination thereof. These techniques can be accurate for individual cells. Moreover, all of these above approaches utilize individual mechanisms in each cell and are directed to cylindrical and prismatic packaged cells. However, in a battery that is typically constructed of multiple cells, charge control becomes more challenging. For example, including a charge control mechanism on each cell can become complex and expensive.
Due to manufacturing differences, cell characteristics can vary slightly from cell to cell and cells may not have identical capacities. Therefore, monitoring the voltage and/or temperature of a group of series connected cells may not always enable controlling the charge to the degree of accuracy that would be desired at the individual cell level. The small voltage and/or temperature changes that can occur at the single cell level at the end of charge may be very difficult to monitor in a large series connected battery. This, therefore, places a need for a very high degree of reproducibility in production and in most cases, the need to pretest and match cells of like capacity in the fabrication of multi-cell batteries in order to employ charge control techniques that function adequately. In some cases, individual cell control, monitoring and bypass is used to enable multi-cell batteries to operate satisfactorily.
Furthermore, as multi-cell batteries are fully discharged, gas can be generated in individual cells that may be forced into reversal as a result of differences in capacity from cell to cell. Conventionally, battery terminal voltage can be utilized to discontinue discharge, but this approach has limitations and may lack accuracy in a series connection of multiple cells. Cell pressure may be used to limit charge and over discharge at the cell level. As a result of manufacturing differences, the ideal system would monitor each individual cell's pressure and use that to limit battery charge and discharge. However, this approach involves the monitoring and controlling pressure of each individual cell in a multi-cell battery, which may be complex, expensive and may not be practical for most battery applications.
The approaches described above with respect to the control of battery charge and/or discharge may add considerable complexity and/or cost to batteries that use large numbers of cells.
Therefore, there is a need in the art for simple and reliable techniques to control the charge of multi-cell batteries. Moreover, there is a need to have a reliable simple technique to discontinue battery discharge prior to excessive buildup of cell pressure and cell reversal during discharge.